In a projection display, a full color image on the screen is achieved by overlaying separate images containing red, green and blue components. These component images are projected by a common projection lens onto a distant screen for viewing. As described in the reference "Applied Photographic Optics," pages 103-111, Sidney F. Ray, Focal Press, 1994, it is a feature of glass that its refractive index changes with the wavelength of light, and a lens may, therefore, have a different magnification for red, green and blue images. Therefore, it is commonly seen that the red, green and blue component image do not exactly overlap especially at the edges of the screen. This is a serious problem for some applications and it is necessary to construct a projection lens of considerable complexity to attempt to remedy this (color convergence) problem.
To minimize this effect, a lens may consist of a number of elements employing different glasses so as to color correct the lens at two or more specific wavelengths. As displays of higher resolution are developed, the demands on the projection lens become more severe.
In applications such as rear projection monitor or TV, the viewer may approach closely to the screen and misconvergence detracts noticeably from the quality of the image. In electronic theater applications, misconvergence at the edges of the screen is objectionable since some viewers may sit significantly to the side of the screen.
FIG. 1 illustrates the imperfect overlapping of, for example, red and blue images at a screen. The dots may be made to overlap perfectly at the center of the screen by adjustment of the position of the objects being imaged, however, there is a displacement (misconvergence) between the different colored dots toward the edge of the screen.
FIG. 2 shows a highly simplified projection system 50 where a target or panel A, 100, is imaged in red light by a projection lens 200 onto a distant screen 300. The outline of the imaging panel 100 is shown as a square 400 on the screen. A second panel B, 140, is imaged in blue light via a dichroic mirror 180 that reflects blue light, but is transparent to red light. In this way, an image 440 of the panel is produced on the screen in proximity to the image 400.
In a projector, the panels 100, 140 are variously described as light valves or spatial light modulators. The panel may be transmissive in type, that is, light is transmitted through the panel or reflective in type where light is reflected from the panel. The light valves are designed to produce a pattern or image within the light valves from an electronic or other source, that is small in size, e.g., one inch in width. This image is projected onto a distant screen 300 with a magnification of order 20 to 200 times or more.
It can be seen that in the example shown in FIG. 2, the size of the images 400 and 440 are slightly different due to the variation in lens strength with color of light. It is the case that the magnifications cannot be equalized by movement of one of the panels 100 or 140 since there is only one position of correct focus for the panels. To vary the magnification, it is necessary to vary the distance from lens 200 to the screen 300 separately for the two panels, however this is not possible. In a projector, red, green and blue images are required and the optical system may include more than two panels that are imaged onto the screen.
It would be highly desirable to provide an apparatus for improving the color convergence of projection display systems that is simple and does not require modification of the projection optical system.